In vehicle fuel control systems, closed-loop control is commonly implemented to control a ratio of air to fuel delivered to an engine. An exhaust oxygen sensor typically senses oxygen content in the engine exhaust. A vehicle control module may continuously adjust fuel to the engine based on the oxygen content information.
Closed-loop fuel control may be based on a switch-type oxygen sensor or, alternatively, on a more expensive proportional sensor that provides proportional equivalence ratio (ER) information. The switch-type sensor cycles essentially to a low or high state when a sensed air-fuel ratio goes above or below a narrow range about a stoichiometric set-point for the sensor. The switch-type sensor signal thus can indicate whether an exhaust stream is rich or lean of stoichiometry. Unlike the proportional sensor, however, the switch-type sensor cannot effectively detect ranges of air-fuel ratios. Because of this lack of proportional ER information, switch-type sensors generally are used in connection with integral-type closed-loop control algorithms.